Aluminum and its alloys are resistant to atmospheric and many other corrosive agents because of the presence of a thin film of aluminum oxide on its surface. If the metal is scratched exposing fresh metal, a thin, tenacious film is rapidly reformed by the reaction of the aluminum with oxygen in the air. Unfortunately the chemical reactiveness which causes aluminum to form this omnipresent oxide film and the resistance to corrosion which this film imparts to the base aluminum, also make aluminum and its alloys very difficult metals on which to apply other metal coatings by electroplating or electroless plating.
Since the large scale production of aluminum began early in this century there has been extensive research into methods for applying adherent coatings to aluminum. Several very successful processes have been developed and large quantities of aluminum alloy parts are electroplated every year for both decorative and functional purposes. The airline and aerospace industries as well as the military use a great deal of this material.
The most extensively used method is the zincate process which dates from the work of Hewitson (U.S. Pat. No. 1,627,900 issued in 1927) which, with improvements, is still the method used for the major portion of the aluminum which to be is plated. In this process, after suitable cleaning and acid dipping to insure a surface free from oils, gross contamination, and variations in the thickness of the oxide film, the aluminum is immersed in a concentrated alkaline solution of an alkali metal zincate (usually sodium zincate), the oxide film is dissolved and replaced with a continuous film of zinc metal deposited by chemical displacement. There are patented variants to the process, e.g., the addition of sodium cyanide or other complexing agents, the addition of other metal salts, etc. designed to make the deposit more adherent or simplify the processing. Basically, however, the process involves the removal of the oxide film and the deposition of the zinc film to prevent its reformation before the aluminum can be plated.
In the case of some electrodeposited metals, such as chromium, it is possible to go directly into the final plating bath, but usually it is necessary to go into a special copper strike bath to avoid interaction of the zinc with subsequent plating baths. This adds to the complexity of the process and increases the possibility of subsequent failure.
If the final metal plating is to be an electroless (autocatalytic) plate such as nickel deposited by chemical reduction using sodium hypophosphite instead of reduction by an externally applied electric current, the zinc acts as a poison to the electroless process making it difficult to initiate plating and is a strong contaminant to the bath causing it to plate at a greatly diminished rate.
Another less widely used process involves the deposition of an immersion tin coating from an alkali stannate bath followed by a bronze strike. A bath of this type is sold by M & T Chemicals of Rahway, N.J. under the name Alstan Process.
Other patented immersion processes are copper from acid solutions (U.S. Pat. No. 1,372,290 issued in 1921 to Hurley), tin from neutral or alkaline solutions (U.S. Pat. No. 1,045,718 issued in 1912 to Marino), iron from a ferrous chloride acid solution (U.S. Pat. No. 2,162,789 issued in 1939 to Raub et al.), antimony from acid chloride solutions (U.S. Pat. No. 2,485,182 issued in 1949 to Arent), brass from cyanide solutions (U.S. Pat. No. 2,496,845 issued in 1950 to Balden et al.), nickel from strongly acidic chloride solution (U.S. Pat. No. 2,746,136 issued in 1956 to Richaud), and heavy metals (e.g., cadmium, zinc, etc.) from fluoride, fluosilicate or fluoborate solutions (U.S. Pat. No. 2,297,241 issued in 1942 to Leonhard Perner).
Besides zincate or stannate baths, it is known to plate iron or aluminum from an acid solution (Grund '789) and nickel from an acid chloride solution (Richaud '136). Ironplating will rust if not processed promptly and has a tendency to produce very rough deposits. Nickel coating according to Richard '136 gives very rough granular deposits for later lead plating wherein the adhesion results from mechanical interlocking of the subsequent lead plate with the nickel. Accordingly, the Richaud process deliberately creates an uneven surface using basic solutions specifically for the purpose of facilitating mechanical adherence of the lead plate. The acid concentration is such that there must be continuous attack on the aluminum surface even after removal of the aluminum oxide. The immersion time in '136 may also serve as a basis for dissolution of the aluminum surface.
Also U.S. Pat. No. 4,360,411 issued in 1982 to Ladet, et al. is limited to aluminum electrical contacts and involves exposure of the aluminum to concentrated acid.
Immersion processes are preferred for preparing aluminum for subsequent plating since in general they require less time and equipment, are less critical to control and have the widest applicability of all the known methods. It has been found that prior immersion solutions do not give uniformly good adhesion on all aluminum alloys. Some are successful with relatively pure aluminum, some were with only casting alloys while others are only successful on wrought alloys.